1. Field of the Invention:
The present invention relates to a claw-pole type stepping motor, and particularly to a stator structure of a claw-pole type stepping motor for realizing the miniaturization and high efficiency of the motor and also the simple and easy manufacture of the motor.
2. Related Art:
In recent years, various devices have been coming out with miniaturization and high efficiency, and this trend keeps going on. However, although sufficient space a s a whole is provided in devices such as a laptop computer, the height space is limited, and high performance characteristics are required for the motor.
A claw-pole type stepping motor is capable of high precision position control with a comparatively simple structure, but it is difficult to reduce the size (diameter) of such a motor. This arises mainly from the formation of the teeth of claw poles.
FIG. 1A is a partially cut away perspective view showing a conventional claw-pole type stepping motor, and FIG. 1B is an exploded perspective view of yokes with pole teeth that are main portions of a stator.
In FIG. 1A, reference numeral 1 denotes a flange for motor attachment, and an outer yoke 2 and an inner yoke 3 are each manufactured by punching and drawing a soft magnetic steel plate, each of the yokes 2 and 3 having at its inner circumferential side a plurality of pole teeth 4 formed by bending. Coils 6 formed by winding magnetic wires around bobbins 5 are each arranged at an equal interval in an annular space between the outer yoke 2 and the inner yoke 3. Respective pole teeth 4 of the outer and inner yokes 2 and 3 are displaced by an electrical angle of 180xc2x0, and from a magnetic circuit together with the coil 6, thus constructing each of an A phase stator 15 and a B phase stator 16. The A phase and B phase stators 15 and 16 thus constructed are coupled together back to back such that their respective pole teeth 4 are displaced by an electrical angle of 90xc2x0, thereby forming a stator assembly 17. A lower flange 7 has a bearing (not shown) at its center and is attached to the outer yoke 2 of the B phase stator 16. Reference numeral 9 represents a rotor magnet, and a sleeve 10 joins the rotor magnet 9 to a shaft 11.
As can be seen in FIG. 1B, the plurality of pole teeth 4 are formed in the outer yoke 2 and inner yoke 3, respectively, by bending the pole teeth after integrally forming them at the time of machining the respective yokes. Accordingly, since the pole teeth 4 are made of the same material of a section corresponding to a hole formed in the center of each of the yokes, it is extremely difficult to arbitrarily set the length and thickness of each pole tooth 4. For example, if the motor diameter is made small, it is difficult to obtain each pole tooth of sufficient length. For this reason, a claw-pole type stepping motor tends not to be mounted in a thin device in which only a motor having a very small diameter can be used.
To solve this problem, a structure where pole teeth are parts manufactured discretely from the yokes has been proposed and an example is disclosed in Japanese Patent Application Laid-open No. 2000-050536. However, since this solution proposes that each individual pole tooth should be manufactured and assembled discretely from other pole teeth and from the yokes, it is not feasible method and structure to adapt for a practical motor, because it requires a great deal of labor and cost to manufacture this type of motor.
Therefore, with a claw-pole type stepping motor of the related art, each of the pole teeth 4 of the outer yoke 2 and inner yoke 3 are integrally formed with their respective yokes. However, in order to secure sufficient space for coils, the smaller a motor diameter becomes the thinner the plate has to be. As a result, from the point of view of strength as well as magnetic characteristics, it is very difficult to form satisfactory pole teeth. For this reason, instead of using a claw-pole type stepping motor, a brushless DC motor is used. However, in this case, it is necessary to add an encoder as a position detector just to control its position. As a result, the structure of the brushless DC motor becomes expensive.
Nevertheless, in an environment surrounding the stepping motor, a high performance magnet such as a rare earth magnet has been developed as the material of a rotor magnet facing the stator, and only if a satisfactory structure of a stator can be found out, will it be possible to provide a motor with improved characteristics, and its appropriate structure is therefore expected.
The present invention has been made in view of the above-described circumstances in the prior art, and has for its aim to provide a claw-pole type stepping motor capable of being mounted in a thin device and being manufactured easily, and at the same time to provide a stator structure capable of dealing with a high output motor.
In order to achieve the above-described aim, according to a main aspect of the present invention, there is provided a claw-pole type stepping motor comprising: a stator in which coils are arranged in association with a yoke unit made of soft magnetic steel and consisting of an inner yoke and an outer yoke, each having a round hole at its center, and a of pole tooth sections made of soft magnetic steel thereby forming a magnetic circuit; and a rotor magnet which is rotatably disposed opposite to the pair of pole tooth sections of the stator, each of the pair of pole tooth sections being prepared discretely from each of the inner and outer yokes by punching out a soft magnetic strip steel plate so as to form a piece having a belt portion and a plurality of pole teeth integrally connected to the belt portion and thereafter rolling into a cylindrical shape.
The above-described stepping motor of the main aspect also has the following subsidiary aspects:
(a) In the stepping motor of the main aspect, each of the pole tooth sections is inserted into the round hole of each of the yokes and is fixed to an inner circumferential surface defining the round hole of each of the yokes.
(b) In the stepping motor of the main aspect, fixing each of the pole tooth sections to an inner circumferential surface defining the round hole of each of the yokes is achieved by press-fitting the pole tooth section into the round hole.
(c) In the stepping motor of the main aspect, fixing each of the pole tooth sections to an inner circumferential surface defining the round hole of each of the yokes is achieved by welding the pole tooth section to the yoke.
(d) In the stepping motor of the main aspect, fixing each of the pole tooth sections to an inner circumferential surface defining the round hole of each of the yokes is achieved by pouring mold resin into a gap in an assembled body of the yoke and the pole tooth section.
(e) In the stepping motor of the main aspect, protuberances facing the inside of the round hole are formed at appropriate places of an inner circumferential surface defining the round hole of each of the yokes, so that the protuberances engage fixedly with engaging portions of each of the pole tooth sections when joining the yokes with the pole tooth sections, respectively.
(f) In the stepping motor of the main aspect, protuberances facing the inside of the round hole are formed at appropriate places of an inner circumferential surface defining the round hole of each of the yokes, and notches are cut in appropriate places of a side of the belt portion of each of the pole tooth sections opposite to another side of the belt portion where the plurality of pole teeth are formed, so that the protuberances engage fixedly with the notches when joining the yokes with the pole tooth sections, respectively.
(g) In the stepping motor of the main aspect, notches are cut in appropriate places of an inner circumferential surface defining the round hole of each of the yokes, and protuberances are formed at appropriate places of a side of the belt portion of each of the pole tooth sections opposite to another side of the belt portion where the plurality of pole teeth are formed, so that the protuberances engage fixedly with the notches when joining the yokes with the pole tooth sections, respectively.
(h) In the stepping motor of the subsidiary aspects (f) and (g), the protuberances and notches are respectively formed at at least two places diametrically opposed so as to engage with each other.
As it is clear from the above aspects, according to the stepping motor of the present invention, the pole teeth of the soft magnetic steel facing the rotor magnet are prepared discretely from each of the inner and outer yokes, and the soft magnetic strip steel plate is punched out to form the pole tooth section having a belt portion and a plurality of pole teeth integrally connected to the belt portion, and the pole tooth section is rolled into a cylindrical shape. As a result, individual dimensions of each of the pole teeth (length, width, plate thickness, etc.) can be arbitrarily set as required by the characteristics of the motor, which can realize a miniature motor that develops high output. Moreover, the present invention makes it possible to reduce the number of parts and to easily assemble the motor.